Recent advancements in industrial and scientific control systems have witnessed a substantial increase in the use of computer-controlled robotic apparatus for the manipulation of workpieces, instruments, components, etc. in place of the human worker or operator. The present generation robot is essentially a mechanical manipulator (usually having six degrees of freedom of available displacement) driven by digital commands from a controlling processor. The closed loop information that is available to the processor usually consists of position and velocity encoder data at the manipulator joints and may also include either current levels at the electrical actuators or pressure levels at the hydraulic actuators. Unfortunately, this amount of information is insufficient to accurately describe the actual location of the end effector of the manipulator in space, such that closed-loop operation of the system has not yet been attainable. Moreover, almost all robots vibrate at a low frequency (from three to ten Hz) and when oscillated by means of human "hand shaking" will vibrate with an amplitude of 3/8" to 1/2". This indicates that the robot arm is not truly accurate under load, so that it cannot be expected to perform precision machining operations, make precision measurements during such operations (automatic inspection), or respond to an independent set of commands from a processor-supplied data base.
One example of the application of robotic systems to state-of-the-art precision machining tasks involves the use of jigs for supporting the manipulator structure. This approach has been employed in the aircraft industry where an expensive precision jig is used to guide a drill bit held by the robot in order to accurately drill hundreds of holes in a composite wing surface. In this system, which undesirably requires a complex and daily calibration, the robot system is used merely to automate the sequencing of the hole-to-hole drilling procedure and to preload the drill so that cutting action is ensured. As such, the robot makes no contribution to the precision hole location layout or its inspection. Moreover, the jig is essentially a barrier to flexible automation since its form is necessarily static, and thereby prevents any response to an arbitrary processor-supplied data base.
Another example of today's state-of-the art involves the precision routing of air frame surface panels that must be cut to .+-.1/16" to .+-.1/32" accuracy. These panels, which must have a large variety of three-dimensional shapes, are pre-cut oversize and formed in a press. The panels must then be cut to final specifications with a high speed router cutter. This is usually accomplished by placing the uncut panel in a jig which has precision surfaces on which concentric (with the router cutter) rollers can roll. Hence, again the robot only provides the preloading and sequencing of the motion of the precision cutter. It is not responsible for any of the accuracy requirements either during the process or during inspection.
One attempt to deal with the problem of drilling in special wing surfaces has been through the use of a special large X-Y-Z table provided with a drilling head developed by Grumman Aerospace. This machine provides the level of accuracy desired and is capable of responding to a broad range of commands. However, this specially configured (nongeneric) drilling table addresses only a special problem of interest. It does not deal with the basic shortcomings of current generation robots, namely the accuracy of positioning the end-effector under load conditions for a wide variety of industrial and scientific applications and configurations.
When analyzing the operations to be performed by a robotic system as a replacement for a human counterpart, it is extremely important to realize that the human system performs its function because of unsurpassed hand-eye coordination. Nonetheless, the human system is probably a very poor model for performing precision operations under load. This fact has not been widely recognized and to a great extent slows the scientific and industrial community's response to this valid need. Note that no human hand is capable of precision measurement or is capable of precision machining operations under load. Because this is true, the human model for robotic manipulators is adequate only for relatively simple repetitive tasks such as pack-and-place, spot welding, painting, surveillance, unloaded assembly, etc. A precision system capable of high loads in real-time operation that is unattainable by the human mode could perform such tasks as precision machining without jigs, precision inspection during operation, multi-level operations under CAD/CAM control, force fit assembly, etc.
Unfortunately, as pointed out above present day robots used in precision assembly are capable of only light loads and performing those tasks only under conditions which are repetitive and unchanging. One contradiction is that high load actuators provide very poor resolution and, vice versa, high resolution actuators provide very low load capacity. This fundamental contradiction appears to be unsolvable in terms of the present state-of-the-art.
An additional consideration is the fact that most manipulator arms are accurate to only about 0.050 inch, which is insufficient for many precision operations such as assembly operations and spatial motion functions (for example, laser cutting and welding operations). Precision requirements will intensify as robotic systems become smaller (micro-robotics) and are applied to such functions as micro-surgery. Generally, deformations under small applied loads in existing commercial systems substantially exceed the static (or repetitive) precision level they can achieve. In addition, the operating environment (temperature, loads, shocks, vibrations, etc.) cannot always be considered invariant, so that not only must the robotic system be capable of precise manipulation, with a narrowing range of tolerances, under load, but it must be adaptable to changing conditions.